Rapton60

I run Affinity Photo 1.5.1 on an Imac 27 ( mid2011 ) with 4GB Ram and clean install with OS Sierra. It is slow, very slow, it takes a minute to change aD800 raw file between personae. It freezes in tone mapping. Do I need to upgrade my Ram memory to solve the problem ? How do I set the performance in Preferences. Open GL and integrated GPU ? Activity monitor shows this. ( program not responding from time to time ) Thanks in advance.